ASTR 101 Lecture 11 Outline of Last Lecture I The Sun a Physics of the sun b Age of the sun II Fission and Fusion III Sun Earth Connections Outline of Current Lecture I Measuring Stars II Stellar Parallax III Luminosity Brightness Distance IV Light and Temperature Current Lecture How do we know how far away the stars are from us Sun is closest Next star is 4 light years away Taking measure of the stars How to measure luminosities temperatures compositions and masses of stars Stellar Parallax If something is far away it does not look like it is moving as much Closer stars look like they have moved from January to July because of Stellar Parallax But really they have stayed in the same place 1 parsec distance 1AU moves 1 arcsecond Not many stars are close enough to measure this We send satellites into space to get closer Parsec a star with a parallax of 1 arcsecond is 1 parsec away from the sun distance is inversely related to parallax Luminosity The total amount of power energy per sec the star radiates into space Apparent brightness in the amount of starlight reaching Earth energy per sec per square meter Luminosity does not equal brightness Distance brightness and luminosity If you know how luminous the star is you know how far away it is Inverse Square Law Brightness 11R2 Stellar Luminosity range 106 Lsun is most luminous stars 10 4 Lsun is least luminous stars Light and Temperature Nearly all large or dense objects emit black body radiation including stars planets and sun An object s blackbody radiation spectrum depends only on one property temperature Hotter objects emit more light Hotter stars are blue cooler stars are red Cecilia Payne Stars mostly made of Hydrogen Helium What causes spectral lines Absorption lines in a star s spectrum tell us how ionized the star s gas is Measuring Stellar Masses You can use Kepler s Third Law to measure star s mass if we can measure the orbital period and separation use gravity to measure mass Eclipsing Binary Star System Orbital plane tilted by 6 degrees with respect to line ofsight Spectroscopic Binary Star System Stars that are too close to be observed individually but whose spectra are separate Ranges Luminosity 10 4 Lsun to 106 Lsun Mass 8 Msun to 100 Msun Temperature 3 000 K to 50 000 K Massive stars are rare Many more low mass stars
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